In U.S. Pat. No. 3,824,413, issued on July 16, 1974 to M. Awipi et al., the use of a semiconductor charge transfer device as a delay line in a recursive oscillator configuration was disclosed. Such an oscillator is characterized by a continuous spectrum of possible resonant frequencies of oscillation. However, in certain technological applications, it is desirable that the resonant frequency of the oscillator be well controlled and stabilized at a single predetermined frequency. It is also desirable in such cases to have an ordered array of such oscillators which can thus provide a corresponding ordered array (discrete spectrum) of mutually different but precisely defined resonant frequencies, for use as local heterodyne oscillators.
In such technological applications as a frequency division multiplexed system for voice communications, for example, each message channel of the system is assigned a discrete segment of the transmitted frequency spectrum. The problem of crosstalk or confusion between adjacent voice communication channels can become intolerable in a channel bank filter receiver ("demultiplexer") in such a system unless the bandpass filtering used for discriminating between one channel and another is sufficiently well-controlled, pure, and sharp. One approach for obtaining a useful channel bank filter receiver involves the use of heterodyne principles, in order to reduce the carrier frequencies to a more convenient and manageable range. Each of such filters in a bank requires a "local oscillator", for heterodyning ("beating down") the channel frequencies to this more manageable range, known as the intermediate frequency (I.F.). Since the requirements of stringent control and complexity (and hence costs) of a passive bandpass filter (for a given sharpness and precision of the passband) vary inversely as .DELTA.f/f.sub.o, where .DELTA.f is the bandpass frequency range and f.sub.o is the center frequency of the band, the heterodyning process of "beating down" the carrier frequency of a signal wave with a local oscillator decreases the value of f.sub.o and hence relaxes the precision requirements on the bandpass filters. However, the local oscillator signal used for the heterodyning process in each channel must be well-controlled as to precision and purity of frequency as well as stability; otherwise, undesired crosstalk or confusion between channels arises. Thus, it is desirable to have an ordered array of such local oscillators, each of whose resonant frequencies is well-controlled, pure, precise, and stable. Moreover, these resonant frequencies should form an ordered array (discrete spectrum) of frequencies differing in equal steps corresponding to the (equal) frequency separations between next adjacent carrier frequency channels, typically a spectrum of frequencies in steps of 4 KHz for voice communications; and each of these local oscillators should have an output characterized by a stabilized sinusoidal frequency profile which is free from any frequency components of any of the other channels, in order to prevent undesirable crosstalk between such channels. However, the use of recursive filters operating as oscillators of the type described for example in U.S. Pat. No. 3,824,413 (issued to Awipi et al. on July 16, 1974) with purely semiconductor charge transfer device (CDT) elements in the feedback path would suffer from the problem of instability and hence crosstalk between channels.